Method for producing thermoelectric elements

ABSTRACT

Thermoelectric elements with excellent thermoelectric characteristics such as Seebeck coefficient thermoelectromotive force and thermal conductivity can be produced by molding a powder of metal or metal alloy as the raw material and then sintering; by using as such raw material, ultra fine powders containing Fe and Si as main components and having a mean particle diameter of 50 to 5,000Å.

This is a continuation of application Ser. No. 07/327,592, filed Mar.23, 1989, U.S. Pat. No. 4,992,235.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing thermoelectricelements and more particularly to a method for efficiently producingthermoelectric elements having excellent thermoelectric characteristicsby controlling the particle diameter of the metal and metal alloy as theraw material.

2. Description of the Related Arts

In recent years, thermoelectric elements to be used in thermoelectricgeneration utilizing the Seebeck effect have been expected to be putinto practice in various fields, and for production of suchthermoelectric elements, various methods have been proposed. A typicalmethod is such that mechanically ground metal and metal alloy are usedas the raw material, and they are molded by compressing by the use ofe.g. a press, sintered and then subjected to thermal treatment toproduce a thermoelectric element having a predetermined shape.

The thermoelectromotive force of a thermoelectric element obtained bythe above conventional method is not always sufficiently high andtherefore, it has been desired to develop a thermoelectric elementhaving a high thermoelectromotive force.

Moreover, since the above mechanically ground metal and metal alloyshave a large particle diameter and are not uniform in shape, the aboveconventional method has problems in that the sintering temperature ishigh, the sintering density after molding is not increased, andintergranular controlling is difficult.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for producinga thermoelectric element which is excellent in thermoelectriccharacteristics such as thermoelectromotive force and thermalconductivity.

The present invention relates to a method for producing a thermoelectricelement by molding metal or metal alloy powders as the raw material andthen sintering the resulting molding, which method is characterized inthat as the raw material, ultra fine powders containing Fe and Si asmain components and having a mean particle diameter of 50 to 5,000 Å areused.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view showing an embodiment of a highfrequency plasma furnace.

FIG. 2 is a graph showing the relation between particle diameter andSeebeck coefficient in the thermoelectric element of Example 1.

FIG. 3 is a graph showing the relation between temperature and Seebeckcoefficient in the thermoelectric element sintered at 850° C. in Example1.

FIG. 4 is a graph showing the relation between temperature and Seebeckcoefficient in the thermoelectric element sintered at 900° C. in Example1.

FIG. 5 is a graph showing the relation between temperature and Seebeckcoefficient in the thermoelectric element sintered at 1,150° C. inExample 1.

FIG. 6 is a graph showing temperature patterns in heat treatment ofExamples 2, 4, 7 and 8, and Comparative Examples 2 and 4.

FIG. 7 is a graph showing temperature patterns in heat treatment ofExamples 3, 5 and 9, and Comparative Examples 3 and 5.

DESCRIPTION OF PREFERRED EMBODIMENTS

The raw material of the thermoelectric element to be used in the presentinvention contains Fe and Si as main components. For example, silicidessuch as Fe_(1-x) Mn_(x) Si₂ (x=0 to 0.15) or Fe_(1-y) Co_(y) Si₂ (y=0 to0.10) can be used. Typical examples are FeSi₂, Fe₀.92 Mn₀.08 Si₂.00 andFe₀.95 Co₀.05 Si₂.00.

In the present invention, as the metal or metal alloy to be used as theraw material, ultra fine particle-formed powder having a mean particlediameter of 50 to 5,000 Å, preferably 100 to 3,000 Å and more preferably100 to 1,000 Å (hereinafter referred to merely as ultra fine particles)is used.

In production of ultra fine particles of metal or metal alloys, commonlyused methods such as the plasma method, the CVD method, the evaporationmethod or the ball mill method can be employed. In particular, theplasma method is preferred.

For the plasma method, high frequency plasma method, arc plasma jetmethod, hybrid plasma method and so forth can be used. In accordancewith these methods, simultaneously with the synthesis of various metalalloys, ultra fine particles can be easily obtained.

FIG. 1 shows an example of a high frequency (RF) plasma furnace forproducing ultra fine particles by the above plasma method. A powderhaving the aforementioned composition is used as the raw material andintroduced along with argon (Ar) gas in a RF plasma furnace 1 where theraw material powder is gasified by generating high temperature plasma of10,000 K in temperature by the use of a high frequency coil 2. In thelower part, the gasified raw material powder is solidified by quenchingwith Ar gas to produce ultra fine particles 3. Concrete conditions forthe plasma treatment are as follows:

    ______________________________________                                        RF power          10 to 50 (kw)                                               Ar flow rate      10 to 50 (liters/min)                                       H.sub.2 flow rate 1 to 10  (liters/min)                                       Amount of raw     1 to 100 (g/min)                                            material supplied                                                             ______________________________________                                    

The above Ar flow rate is an amount of containing Ar supplied along withthe raw material, and Ar supplied from other two points. H₂ is suppliedto the RF plasma furnace 1 and acts to control the temperature of argonplasma and to stabilize the plasma.

By producing ultra fine particles by subjecting the raw material to theplasma treatment, ultra fine particles having a mean particle diameterof 50 to 5,000 Å can be easily produced, and the particle diameter canbe much smaller as compared with the conventional thermoelectricelements in which the particle diameter is about 1 μm. By application ofthe plasma treatment for powdering of the raw material, the phase ofultra fine particle can be made a semiconductor phase alone or a mixedphase of a semiconductor phase and a metal phase. In the mixed phase,the ratio of the semiconductor phase (β) to the metal phase (α+ε) ispreferably from 9:1 to 1:1, and more preferably from 6:1 to 2:1. Bymaking fine particles of sintered material constituting thethermoelectric element, smaller than the conventional ones, and furtherby producing fine particles of a semiconductor phase or ultra fineparticles of a mixed phase of a semiconductor phase and a metal phase,there can be obtained a thermoelectric element having such highcharacteristics in that the thermoelectromotive force and specificresistance are increased, and thermal conductivity is decreased. Inproducing fine particles by application of the plasma treatment, thesintering and heat treatment steps can be simplified as compared withstep of the conventional grind using a ball mill. Moreover, since ultrafine particles are used as the raw material for the thermoelectricelement, sintering can be carried out at a lower temperature and theenergy cost can be reduced.

By changing the type of the raw material or conditions for the plasmatreatment, particularly the RF power in the above plasma treatment, theparticle diameter of ultra fine particles to be produced can becontrolled and the phase of the ultra fine particle can be made not onlya semiconductor phase but also a mixed phase of a semiconductor phaseand a metal phase. If only a powder having a large particle diameter asobtained by the usual mechanical powdering is used, the resultingthermoelectric element has an insufficiently low thermoelectromotiveforce, and the objects of the present invention cannot be attained.

In the method of the present invention, characteristics of thethermoelectric element can be improved by subjecting the above ultrafine particles of metal and metal alloy as the raw material for thethermoelectric element in an atmosphere of reducing gas or reducing gasand halogen or halogen compound-containing gas at the time of sinteringor at an appropriate time before the sintering, more specifically (1) inthe state of ultra fine particles before molding, (2) at the time ofmolding, or (3) at the time of sintering.

During the heat treatment in the above reducing gas atmosphere, theamount of oxygen contained in the raw material is controlled to adjustthe amount of oxygen as a. dopant to a suitable one, thereby increasingthermoelectric characteristics. As the reducing gas, various gases canbe used. Of these gases, H₂, CO, SiH₄, SiH₆, GeH₄ and the like arepreferably used. This reducing gas is introduced into the heat treatmentstep along with a carrier gas comprising generally used inert gases suchas argon (Ar). The reducing gas is used alone or as a mixture. In a casewhere the carrier gas is Ar, the mixing ratio of reducing gas andcarrier gas is as follows: H₂ /Ar=2 to 15%, CO/Ar=2 to 10%, (H₂+SiH₄)/Ar=5 to 10+5 to 10 (%), (H₂ +SiH₆)/Ar=2 to 6+5 to 10 (%), (H₂+GeH₄)/Ar=5 to 15+5 to 10 (%).

The heat treatment of ultra fine particles is carried out at atemperature of 400° to 1,150° C. under a pressure of 0 to 10 kg/cm² Gfor 7 to 60 hours while feeding the above reducing gas, for example, ata rate of 5 to 100 ml/min per gram of the raw material in the case ofH₂. Conditions are determined appropriately, depending on the type, theparticle diameter and the oxygen content of the raw material and soforth.

In the heat treatment of ultra fine particles of the raw material in amixed gas atmosphere obtained by adding halogen, e.g. F₂, I₂ or Br₂, ora halogen compound, e.g. HF or CF₄ to the above reducing gas, the oxygencontent of the raw material is controlled, as in the above case,.and atthe same time thermoelectric characteristics can be improved byintroducing halogen into the raw material. Various halogens or halogencompounds can be used. The amount of halogen or halogen compound isdetermined appropriately in depending on the type of the raw materialand so forth as in the case of the reducing gas. For example, when thehalogen compound gas is HF, the mixing amount is suitably such that(HF+H₂)/Ar=1 to 5+5 to 10 (%).

The action of the heat treatment in an atmosphere of the above reducinggas or a mixed gas of the reducing gas and halogen or halogen compoundgas will hereinafter be explained.

Super fine particles as the raw material of the thermoelectric elementusually contain oxygen in a large amount as high as 2.0 to 15 wt %. Thestate in which oxygen is contained can be classified into four types:(1) surface adsorption; (2) surface oxide film; (3) inter-latticesubstitution; and (4) inter-lattice invasion. Of these types, oxygen inthe lattice acts as a dopant in the thermoelectric characteristics.However, in some cases, oxygen on the surface in types (1) and (2)combines with Fe or Si of the raw material at the subsequent step, e.g.the sintering step, precipitating as a mono-phase oxide, and does notact as a dopant. For this reason, the unnecessary oxygen is removed orreacted with Fe or Si of the raw material by application of the heattreatment using the above reducing gas so as to produce Fe-0, Si-0, etc.in the effective form as a dopant, thereby improving thermoelectriccharacteristics. Moreover, by adding halogen or a halogen compound tothe reducing gas having the aforementioned action, the halogen, e.g.fluorine is caught in the raw material and enters the inside of thelattice, or replaces Si in the lattice and effectively acts as a dopant,thereby improving thermoelectric characteristics.

By changing the oxygen contained in the raw material into a formeffective as a dopant and at the same time, removing excessive oxygen,and further introducing halogen acting as a dopant into the rawmaterial, the anion dopant amount of the raw material can be controlled,thereby improving thermoelectric characteristics.

In the method of the present invention, a thermoelectric element can beproduced by mixing a powder of metal or metal alloy (hereinafterreferred to merely as powder) having a mean particle diameter of 1 to 5μm, preferably 2 to 3 μm with the above ultra fine particles. As thispowder, the same metal or metal alloy as that of the above ultra fineparticle is powdered and used. In powdering the metal or metal alloy,mechanical powdering methods conventionally in general use, such as theatomizing method, the liquid quenching method, the jet mill method, theball mill method, or the stamp mill method can be employed.

Ultra fine particles and powder of the metal or metal alloy to be usedas the raw material of the thermoelectric element are molded aftermixing.

The mixing ratio of ultra fine particles to powder is not critical. Ingeneral, they are compounded in a range such that the powder is 97 to 50wt % and the ultra fine particles are 3 to 50 wt %, and preferably thepowder is 95 to 60 wt % and the ultra fine particles are 5 to 40 wt %.

The mixing ratio is determined appropriately within the suitable rangedepending on the type of raw material, the particle diameter of eachcomponent, the molding pressure, the shape of the molding and so forth.

Although the ultra fine particles and the powder can be mixed by variousmethods, it is difficult to sufficiently mix them by mechanical stirringand, therefore, it is preferred that they be thoroughly mixed by the useof a dispersing agent, a binder, or ultrasonic waves. For example, theultra fine particles and the powder can be sufficiently uniformly mixedby pre-mixing them with a stirring apparatus, e.g. a ball mill,dispersing the resulting mixture in solution of e.g. polyvinyl alcohol,an ethyl vinyl acetate copolymer, stearic acid, colloidal paraffin (oilsand fats), or paraformaldehyde to form a gel or sol, and thenevaporating solvent from the gel or sol by heating while applyingultrasonic waves to form into a granular form.

By mixing the ultra fine particles and the powder, the slight clearancebetween particles of the powder is filled with the ultra fine particlesat the time of molding and thus particle boundary or particle growth canbe controlled. As a result, the sintering density is increased and atthe same time, the clearance is decreased and, therefore, there can beobtained a sintered material which is dense and in which the particleboundary is controlled, and in particular the thermal conductivity isdecreased, and the thermoelectric characteristics of the thermoelectricelement obtained can be increased.

In the method of the present invention, the above prepared powder ismolded, sintered and further is subjected to heat treatment, ifnecessary, according to the usual method.

That is, the molding step, the sintering step or the subsequent heattreatment step can be carried out in the same manner as in theconventional method for producing thermoelectric elements.

For example, molding can be carried out by compressing under a pressureof several hundred kilograms to several tons per square centimeter bythe use of a compression molding machine. Sintering can be carried outby heating the molded material at a high temperature of 800° to 1,200°C. for several hours. In addition, heat treatment is carried out, ifnecessary, by heating the molded material after sintering at atemperature of 700° to 900° C. for several hours. These treatments arecarried out under suitable conditions determined depending on the typeand form of metal or metal alloy as the raw material and so forth, andare not critical in such conditions.

The present invention is described in greater detail with reference tothe following examples.

EXAMPLE 1

Iron silicide FeSi of metal phase (Fe:Si:(Co or Mn)=(0.85 to0.995):2.05:(0.005 to 0.15)) was used as the powder. This powder wassupplied to an RF plasma furnace and was subjected to plasma treatmentunder the conditions as shown below to produce ultra fine particles. Inthese ultra fine particles, the particle diameter was within the rangeof 50 to 5,000 Å, and the ratio of metal phase (α,ε) to semiconductorphase (β) was (0 to 1):(0.3 to 10.0).

    ______________________________________                                        RF power          10 to 100                                                                              (kW)                                               Ar flow rate      10 to 100                                                                              (liters/min)                                       H.sub.2 flow rate 1 to 10  (liters/min)                                       Amount of feed    1 to 100 (g/min)                                            supplied                                                                      ______________________________________                                    

In this example, the concrete treating conditions were set as follows:RF power=35 (kW); Ar flow rate=30 (liters/min); H₂ flow rate=2(liters/min); and amount of feed supplied=5 (g/min).

The FeSi₂ eutectic alloy ultra fine particles thus obtained (α+ε:β=1:3,mean particle diameter: 250 to 700 Å were subjected to compressionmolding, sintering (in vacuum, 850° to 1,150° C, 1 to 5 hours) and heattreatment (800° to 850° C., 10 to 24 hours) to produce N type and P typethermoelectric elements.

In connection with a thermoelectric element produced using ultra fineparticles in which the ratio of mixed phase, (α+ε;β=1:3) was 3.0 (whichwas produced by changing the plasma treatment conditions), a Seebeckcoefficient vs. mean particle diameter (calculated with T.E.M. Method)at a temperature of 600 K was measured, and the results are shown inFIG. 2. A Seebeck coefficient of a thermoelectric element produced atvaried sintering temperatures was measured, and the results are alsoshown in FIG. 2. In FIG. 2, Curve A indicates a relation betweenparticle diameter and Seebeck coefficient when sintering was carried outat 850° C. for 24 hours; Curve B indicates a relation between particlediameter and Seebeck coefficient when sintering was carried out at 900°C. for 24 hours; and Curve C indicates a relation between particlediameter and Seebeck coefficient when sintering was carried out at1,150° C. for 24 hours. All sintered materials were subjected to heattreatment in vacuum at 800° C. for 200 hours.

A relation between temperature and Seebeck coefficient in thethermoelectric element made of each particle diameter sintered at 850°C. was measured, and the results are shown in FIG. 3. Similarly, arelation between temperature and Seebeck coefficient in thethermoelectric element sintered at 900° C. is shown in FIG. 4, and arelation between temperature and Seebeck coefficient in thethermoelectric element sintered at 1,150° C. is shown in FIG. 5.

COMPARATIVE EXAMPLE 1

Using the same material as used in Example 1: a thermoelectric elementwas produced by the conventional powder metallurgical method. Rawmaterial was metal phase. This conventional thermoelectric element wasmeasured for Seebeck coefficient, and the results are shown in FIGS. 3to 5 as Curve D.

EXAMPLE 2

As the raw material, ultra fine particles having the composition andparticle diameter shown in Table 1 were prepared by the plasma methodusing a high frequency. Argon (Ar) was used as a carrier gas, and amixed gas of H₂ and Ar (H₂ /Ar=10/90 (%)) was introduced at a rate of2,000 ml/min per kilogram of the raw material. In the reducingatmosphere comprising the mixed gas, the first heat treatment wascarried out at 700° C. for 10 hours and then the second heat treatmentwas carried out under a reduced pressure of 10⁻⁵ torr at 1,150° C. for 3hours, and subsequently, the third heat treatment was carried out at800° C. for 80 hours. A temperature pattern of these heat treatments isshown in FIG. 6.

Subsequently, ultra fine particles after the heat treatments were moldedunder a pressure of 2 ton/cm² by the use of a compression moldingmachine, sintered at 1,150° C. for 10 hours, and further were subjectedto heat treatment at 800° C. for 80 hours to produce a thermoelectricelement. This thermoelectric element was measured for thermoelectriccharacteristics at 800° C. The results are shown in Table 1.

EXAMPLE 3

A thermoelectric element was produced in the same manner as in Example 2except that the heat treatment was carried out according to thetemperature pattern shown in FIG. 7 while introducing mixed gas of H₂and SiF₄ in the formulation of (H₂ +SiF₄)/ Ar=(5+5)/90 at a rate of2,000 ml/min per kilogram of the raw material. This thermoelectricelement was measured for thermoelectric characteristics. The results areshown in Table 1.

EXAMPLE 4

As the ultra fine particles, ultra fine particles having the formulationand mean particle diameter shown in Table 1 were used, and heattreatment was applied at a temperature pattern shown in FIG. 6 whileintroducing mixed gas of H₂ and Ar in the formulation that H₂ /Ar=10/90(%) at a rate of 3,000 ml/min per kilogram of the raw material.Thereafter, molding, sintering and heat treatment were carried out inthe same manner as in Example 2 to produce a thermoelectric element.This thermoelectric element was measured for thermoelectriccharacteristics. The results are shown in Table 1.

COMPARATIVE EXAMPLE 2

A thermoelectric element was produced in the same manner as in Example 4except that the powder as the raw material was produced by the powdermetallurgical method. Raw material was metal phase. This thermoelectricelement was measured for thermoelectric characteristics. The results areshown in Table 1.

EXAMPLE 5

A thermoelectric element was produced in the same manner as in Example 4except that heat treatment was carried out at the temperature patternshown in FIG. 7 while introducing mixed gas of H₂ and SiF₄ in theformulation of (H₂ +SiF₄)/Ar=(5+5)/90 (%) at a rate of 3,000 ml/min perkilogram of the raw material. Raw material was metal phase. Thisthermoelectric element was measured for thermoelectric characteristicsin the same manner as in Example 4. The results are shown in Table 1.

COMPARATIVE EXAMPLE 3

A thermoelectric element was produced in the same manner as in Example 5except that the powder as the raw material was produced by the powdermetallurgical method. Raw material was metal phase. This thermoelectricelement was measured for thermoelectric characteristics in the samemanner as in Example 5. The results are shown in Table 1.

EXAMPLE 6

As the raw material, metal alloy ultra fine particles having theformulation and mean particle diameter shown in Table 1 were prepared bythe high frequency plasma method. They were molded under a pressure of 2ton/cm² and presintered at 700° C. for 10 hours in a reducing atmosphereof H₂ /Ar=10/90 (%) with argon (Ar) as a carrier gas which had beenintroduced at a rate of 2,000 ml/min per kilogram of the raw materialand then heat treatment was carried out under a reduced pressure of 10⁻⁵torr at 1,150° C. for 3 hours and then at 800° C. for 80 hours.

The thermoelectric element thus obtained was measured for thermoelectriccharacteristics at 800° C. The results are shown in Table 1.

EXAMPLE 7

A thermoelectric element was produced in the same manner as in Example 4except that the heat treatment of the ultra fine particles in thereducing gas atmosphere was not carried out. This thermoelectric elementwas measured for thermoelectric characteristics in the same manner as inExample 4. The results are shown in Table 1.

EXAMPLE 8

As the raw material, ultra fine particles having the formulation andmean particle diameter shown in Table 2 were used. They were subjectedto heat treatment at the temperature pattern shown in FIG. 6 whileintroducing H₂ in the formulation of H₂ /Ar=10/90 (%) at a rate of 1,800ml/min per kilogram of the raw material. Thereafter, molding, sinteringand heat treatment were carried out in the same manner as in Example 2to produce a thermoelectric element. This thermoelectric element wasmeasured for thermoelectric characteristics. The results are shown inTable 2.

COMPARATIVE EXAMPLE 4

A thermoelectric element was produced in the same manner as in Example 8except that the powder as the raw material was produced by the powdermetallurgical method. Raw material was metal phase. This thermoelectricelement was measured for thermoelectric characteristics in the samemanner as in Example 8. The results are shown in Table 2.

EXAMPLE 9

A thermoelectric element was produced in the same manner as in Example 8except that the heat treatment was carried out at the temperaturepattern shown in FIG. 7 while introducing mixed gas of H₂ and HF in theformulation of (H₂ +HF)/Ar=(5+5)/90 at a rate of 1,000 ml/min perkilogram of the raw material in the reduction treatment (heat treatment)of the ultra fine particles as the raw material. This thermoelectricelement was measured for thermoelectric characteristics in the samemanner as in Example 8. The results are shown in Table 2.

COMPARATIVE EXAMPLE 5

A thermoelectric element was produced in the same manner as in Example 9except that the powder as the raw material was produced by the powdermetallurgical method. Raw material was metal phase. This thermoelectricelement was measured for thermoelectric characteristics in the samemanner as in Example 9. The results are shown in Table 2.

EXAMPLE 10

Fe₀.92 Mn₀.08 Si₂.00 as the raw material was ground to ultra fineparticles having a mean particle diameter of about 500 Å by the highfrequency plasma method, and at the same time, a predetermined amount ofFe and Si, and Mn element as a dopant were dissolved by application ofhigh frequency, and then gradually cooled. The ingot thus obtained wassliced and ground with a stamp mill to produce particles (powder). whichhad a mean particle diameter of 2 to 3 μm and were not uniform in shape.Then, 20 wt % of the ultra fine particles and 80 wt % of the powder weremixed.

In this mixing, preliminary mixing was carried out for 3 hour at 700 rpmby the use of a high speed ball mill, and to the resulting mixed powder,a 1 wt % aqueous solution of PVA (polyvinyl alcohol; molecular weight:less than 1,500) was added to form a gel or sol.

The sol or gel was heated while applying ultrasonic waves and granulatedto produce granules having a water content of 10 to 20 wt % and a meanparticle diameter of 0.5 mm.

The mixed powder in the granular form was molded under a pressure of 2ton/cm² by the use of a compression molding machine, sintered by heatingin a vacuum at 1,150° C. for 3 hours, and subjected to heat treatment at800° C. for 30 hours to produce a thermoelectric element. Thisthermoelectric element was measured for thermoelectric characteristics.Tee results are shown in Table 3.

COMPARATIVE EXAMPLE 6

A thermoelectric element was produced in the same manner as in Example10 except that as only powders having a mean particle diameter of 2 to 3μm produced in the same manner as in Example 10 were used for the rawmaterial. This thermoelectric element was measured for thermoelectriccharacteristics. The results are shown in Table 3.

EXAMPLE 11

Using Fe₀.97 Mn₀.03 Si₂.00 as the raw material, both ultra fineparticles and powder were produced in the same manner as in Example 10.They were then mixed, molded, sintered and heated in the same manner asin Example 10 to produce a thermoelectric element. This thermoelectricelement was measured for thermoelectric characteristics in the samemanner as in Example 10. The results are shown in Table 3.

COMPARATIVE EXAMPLE 7

A thermoelectric element was produced in the same manner as in Example11 except that only the powder was used as the raw material. Thisthermoelectric element was measured for thermoelectric characteristicsin the same manner as in Example 11. The results are shown in Table 3.

EXAMPLE 12

A thermoelectric element was produced in the same manner as in Example10 except that a mixture of 5 wt % of the ultra fine particles and 95 wt% of the powder was used. This thermoelectric element was measured forthermoelectric characteristics. The results are shown in Table 3.

EXAMPLE 13

A thermoelectric element was produced in the same manner as in Example10 except that a mixture of 91 wt % of the ultra fine particles and 9 wt% of the powder was used. This thermoelectric element was measured forthermoelectric characteristics in the same manner as in Example 10. Theresults are shown in Table 3.

                                      TABLE 1                                     __________________________________________________________________________                                  Composition of Fine                                    Fine Particle          Particles after                                        Composition                                                                              Mean  Gas for                                                                             Heat Treatment                                                                           Thermoelectric Characteristics              (Fe.sub.1-x Mn.sub.x Si.sub.2-y O.sub.y F.sub.z)                                         Particle                                                                            Heat  (Fe.sub.1-x Mn.sub.x Si.sub.2-y O.sub.y                                       F.sub.z)   Electromotive                                                                         Resistance                                                                           Output P*             No.    x   y   z  Diameter                                                                            Treatment                                                                           x   y   z  Force E (V)                                                                           (Ω)                                                                            (W)                   __________________________________________________________________________    Example 2                                                                            0   0.18                                                                              0  700                                                                              Å                                                                            H.sub.2                                                                             0   0.072                                                                             0  0.22    0.42   0.12                  Example 3                                                                            0   0.18                                                                              0  700                                                                              Å                                                                            H.sub.2 + SiF.sub.4                                                                 0   0.081                                                                             0.03                                                                             0.21    0.46    0.096                Example 4                                                                            0.08                                                                              0.11                                                                              0  700                                                                              Å                                                                            H.sub.2                                                                             0.08                                                                              0.052                                                                             0  0.27    0.41   0.18                  Comparative                                                                          0.08                                                                              0   0  3  μm                                                                            H.sub.2                                                                             0.08                                                                              0   0  0.24    0.55   0.10                  Example 2                                                                     Example 5                                                                            0.08                                                                              0.11                                                                              0  700                                                                              Å                                                                            H.sub.2 +  SiF.sub.4                                                                0.08                                                                              0.33                                                                              0.04                                                                             0.29    0.38   0.22                  Comparative                                                                          0.08                                                                              0   0  3  μm                                                                            H.sub.2 + SiF.sub.4                                                                 0.08                                                                              0   0  0.24    0.52   0.11                  Example 3                                                                     Example 6                                                                            0   0.18                                                                              0  700                                                                              Å                                                                            H.sub.2                                                                             0   0.072                                                                             0  0.24    0.41   0.14                  Example 7                                                                            0.08                                                                              0.11                                                                              0  700                                                                              Å                                                                            --    0.08                                                                              0.10                                                                              0  0.23    0.56    0.094                __________________________________________________________________________     *Calculated from the equation: P(watt) = V.sup.2 /R.                     

                                      TABLE 2                                     __________________________________________________________________________                                  Composition of Fine                                    Fine Particle          Particles after                                        Composition                                                                              Mean  Gas for                                                                             Heat Treatment                                                                           Thermoelectric Characteristics              (Fe.sub.1-x Co.sub.x Si.sub.2-y O.sub.y F.sub.z)                                         Particle                                                                            Heat  (Fe.sub.1-x Co.sub.x Si.sub.2-y O.sub.y                                       F.sub.z)   Electromotive                                                                         Resistance R                 No.    x   y   z  Diameter                                                                            Treatment                                                                           x   y   z  Force E (V)                                                                           (Ω)                    __________________________________________________________________________    Example 8                                                                            0.05                                                                              0.13                                                                              0  500                                                                              Å                                                                            H.sub.2                                                                             0.05                                                                              0.044                                                                             0  0.28    0.22                         Comparative                                                                          0.05                                                                              0   0  3  μm                                                                            H.sub.2                                                                             0.05                                                                              0   0  0.24    0.32                         Example 4                                                                     Example 9                                                                            0.05                                                                              0.13                                                                              0  500                                                                              Å                                                                            H.sub.2 + HF                                                                        0.05                                                                              0.025                                                                             0.04                                                                             0.29    0.31                         Comparative                                                                          0.05                                                                              0   0  3  μm                                                                            H.sub.2 + HF                                                                        0.05                                                                              0   0  0.24    0.32                         Example 5                                                                     __________________________________________________________________________

                                      TABLE 3*                                    __________________________________________________________________________    No.    α.sup.2 · σ · 10.sup.3 (W/K.sup.2               m)*.sup.1   K(W/K · m)*.sup.2                                                             Z · 10.sup.3 (K.sup.-1)*.sup.3           __________________________________________________________________________    Example 10                                                                           1.1         2.7      0.41                                              Comparative                                                                          1.0         3.5      0.29                                              Example 6                                                                     Example 11                                                                           0.8         2.5      0.32                                              Comparative                                                                          0.8         3.5      0.23                                              Example 7                                                                     Example 12                                                                            1.04        3.44    0.30                                              Example 13                                                                            1.06       3.4      0.31                                              __________________________________________________________________________     *Measured at a temperature of 670 K..                                         *.sup.1 α: Seebeck coefficient (V/K)                                    σ: Specific resistance (Ω.sup.-1 m.sup.-1)                        *.sup.2 K: Thermal conductivity (W/K · m)                            *.sup.3 Figure of merit (K.sup.-1) Z = α.sup.2 · σ/K

As Z is larger, thermoelectric characteristics (efficiency of conversionof heat energy into electric energy) are greater.

What is claimed is:
 1. A method for producing thermoelectric elementscomprising molding powders of metal alloy as the raw material and thensintering, wherein said powders of metal alloy are ultra fine powderscontaining Fe and Si as main components and having a mean particlediameter of 50 to 5,000 Å obtained by subjecting the metal alloy toplasma treatment.
 2. The method as claimed in claim 1, wherein the ultrafine powders are subjected to heat treatment in a reducing gasatmosphere.
 3. The method as claimed in claim 2, wherein the ultra finepowders are mixed phase of semiconductor phase and metal phase.